Theoretical evidence for the sensitivity of charge-rearrangement-enhanced x-ray ionization to molecular size

Abstract

It was recently discovered that molecular ionization at high x-ray intensity is enhanced, in comparison with that of isolated atoms, through a phenomenon called CREXIM (charge-rearrangementenhanced x-ray ionization of molecules). X-ray absorption selectively ionizes heavy atoms within molecules, triggering electron transfer from neighboring atoms to the heavy atom sites and enabling further ionization there. The present theoretical study demonstrates that the CREXIM effect increases with the size of the molecule, as a consequence of increased intramolecular electron transfer from the larger molecular constituents attached to the heavy atoms. We compare x-ray multiphoton ionization dynamics of xenon, iodomethane, and iodobenzene after interacting with an intense x-ray pulse. Although their photoionization cross sections are similar, iodomethane and iodobenzene molecules are more ionized than xenon atoms. Moreover, we predict that the average total charge of iodobenzene is much larger than that of iodomethane, because of the large number of electrons in the benzene ring. The positive charges transferred from the iodine site to the benzene ring are redistributed such that the higher carbon charges are formed at the far end from the iodine site. Our first-principles calculations provide fundamental insights into the interaction of molecules with x-ray free-electron laser (XFEL) pulses. These insights need to be taken into account for interpreting and designing future XFEL experiments.X-ray free-electron lasers (XFELs) have opened new horizons for studying the structure and dynamics of matter with femtosecond temporal and atomic spatial resolution [1][2][3]. With the rapid advance in XFEL technology, the peak intensity already exceeds 10 20 W/cm 2 in the hard x-ray regime [4]. At extremely high x-ray intensity, it is crucial to understand the nonlinear interaction of atoms [5][6][7][8][9], molecules [10][11][12][13][14][15][16][17], and clusters [18-20] with xray fields. The interaction between matter and intense x-ray pulses can be described by the multiphoton multiple ionization dynamics model. An x-ray photon ionizes an atomic inner-shell electron, followed by relaxation processes. During the intense x-ray pulse the photoionization and accompanying processes repeat sequentially. This ionization model has been verified for atomic systems with a series of gas-phase XFEL experiments [5,7,8].When a molecule is irradiated by x rays, they mainly interact with heavy atoms within the molecule, because their photoionization cross section is much higher than that of light atoms. It had been widely believed that ionization dynamics of a molecule exposed to XFEL irradiation is similar to that of an isolated atom whose cross section is comparable to the molecule [12][13][14][15]. This expectation turned out to be valid in the low-fluence regime. For example, the total molecular ionization of methylselenol (CH 3 SeH) at low x-ray intensity, where single-photon absorption is dominant, was found to be similar to the atomic ionizat...

Strategies for solving the excited-state self-consistent-field problem for highly excited and multiply ionized states

“…Among the C atoms, the atom C1, which is next to the O atom, stands out by having a considerably lower final charge compared to the other carbon atoms. This observation is similar to the previously reported polarization effect in iodobenzene [31], where the electrons in the benzene ring tend to be distributed so as to shield the larger charge of the neighboring heavy absorbing atom (here O). The charges of the hydrogen atoms, shown in Fig.…”

“…In reality, the molecule undergoes rapid structural deformations due to the high charge in the molecule, eventually leading to a Coulomb explosion. From our previous simulations [28,31], we know that due to the high ionization rate and the short pulse duration (here 10 fs FWHM) these structural deformations have very little impact on the resulting ionization dynamics, such that the final atomic charges are barely affected. The calculated partial charges from a fixed-geometry calculation thus serves as a good approximation to the resulting ion fragment charges.…”

“…To understand the interaction of intense x-ray light with matter, one must consider that a molecule interacting with the x-ray pulse may undergo a series of photoionization, fluorescence, and Auger decay processes eventually causing the ionization of a significant fraction of the electrons. It has been demonstrated that these ionization dynamics can be modeled via rate equations for the time-dependent populations of electronic configurations assuming multiple consecutive photoionization and electronic relaxation (Auger decay and fluorescence) steps [16,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. To that end the relevant cross sections and rates have to be provided.…”

“…To that end, we seek to calculate excited electronic states by employing a mean-field approximation directly solving the self-consistent-field (SCF) equation for excited electronic configurations [33][34][35][36][37][38][39][40][41][42]. In our previous works [16,25,28,31,43], we employed the excited state orbitals obtained with the Hartree-Fock-Slater (HFS) [44] method as implemented in the XMOLECULE toolkit [25,43] to obtain cross sections and rates. By solving the SCF equations for each electronic configuration, the molecular orbitals are adapted to the new electronic configuration after each photoionization, fluorescence, or Auger decay step.…”

Spectroscopic Signature of Chemical Bond Dissociation Revealed by Calculated Core-Electron Spectra

“…16,24,25 The interplay of molecular effects accompanying core ionization give rise to valence rearrangement between the different atoms that are of strong fundamental interest. [26][27][28][29] Investigating the transition from an intact molecule to a dissociated fragment via femtosecond-resolved XPS and AES holds the potential for elucidating important aspects of these mechanisms.…”